This invention relates to an alarm system for detecting the breaching of security of an installation and, in particular, the detecting of the breaking of glass enclosures.
The present day need for security systems is an ever increasing one with the statistics relating to forcible entry, burglary and the like continually rising. As a result a multiplicity of systems and devices designed to sense unwanted intrusions and physical damage have appeared in the marketplace. The problems of security are particularly severe in installations wherein the premises or goods are exhibited to the public. The viewing medium used in these structures is typically glass which can be readily shattered and the premises entered.
To achieve a measure of security in glass-enclosed environments, the use of aluminum foil conductors mounted on the inner surface of the glass enclosures is presently utilized in many establishments. The system when activated relies on the uninterrupted flow of a direct current through these conductors. The breaking of the glass enclosure severs the tape, interrupts the current flow and triggers the alarm. Thus, each window and glass enclosure must be provided with the properly mounted and appropriately located aluminum foil strips and the system is expensive to install as well as detracting from the appearance of the structure and the goods viewed therethrough.
Also, the foil conductors are fragile and often severed by window washers or other employees thus requiring repair or replacement. This type of accident is usually discovered and corrected for without generating a false alarm. However, difficulties in matching temperature coefficients of expansion of glass and foil often open circuit the conductor and this is not noticed until the alarm is needed or activated. As a result, considerable interest has been generated in alternate alarm systems which require lower maintenance and do not detract from the display.
One system that has been utilized as an alternative to conductive foil strips employs mercury switches physically attached to the glass enclosure in a position such that the shock associated with the breaking of the glass enclosure is transmitted directly to the switch. The shock alters the attitude of the mercury in the switch and either opens or closes the electrical circuit to activate the alarm. This system has been found to encounter difficulties in adequately bonding the switches to the window so that the shock waves are transmitted to the switch.
Attempts have also been made to utilize remotely located sensors that are activated by the sound waves generated by the breaking of the glass enclosures. Systems of this type have not generally been satisfactory in environments wherein background noise is present or likely to be encountered since the extraneous noises often activate the alarm.
Accordingly, the present invention is directed to the provision of an alarm system for structures having glass enclosures wherein the sensors are remotely located. Further, the sensors pick up the acoustic waves generated by the initial breaking of the glass enclosure and, following an interval of low noise, the subsequent acoustic waves generated by the broken glass coming to rest at its landing place in order to essentially eliminate false alarm signals being generated.
The present alarm system is characterized by its ability to generate an electrical alarm signal from a pattern of acoustic signals such as that resulting from the breaking of a glass enclosure in environments wherein extraneous noises are likely occurrences. This sensitivity to actual conditions is due in part to a series of timing, magnitude and frequency content determinations performed by the present invention.